1. Field of Invention
The present invention relates to an injection molding machine. In particular, the present invention is directed to an injection molding machine providing a nozzle touch member provided to open and close a tip opening of a heating cylinder so that a plasticizing-measuring process can be conducted without touching a nozzle on a sprue of a mold or laying it open.
2. Description of Related Art
In general, a single molding cycle process in an injection molding machine includes a plasticizing-measuring process. In the plasticizing-measuring process, resin material is reserved in a front portion of a heating cylinder while the resin material in the heating cylinder is kneaded and melted/plasticized by a screw that rotates, for example, on an axis of the heating cylinder. The resin material is reserved and measured in the front portion of the heating cylinder as the screw generates back pressure while it retreats in an axial direction to a given position. Next, the screw moves forward in the axial direction from the retreated position so that molten/plasticized and measured resin material is injected into a cavity of a mold from a nozzle provided in a tip of a heating cylinder.
The temperature at which the resin materials are melted/plasticized in the plasticizing-measuring process is dependent on the type of resin material being used, and the particular molding conditions, including but not limited to, the melting/plasticizing temperature, the configuration of the molded product and the temperature of the mold.
In the above-mentioned process, "drooling" can occur wherein resin material that is melted/plasticized and reserved in the front portion of the heating cylinder leaks from the tip of the nozzle. If drooling occurs, various problems may result, including but not limited to, a shortage of molten resin material, contamination of surroundings due to dripping molten resin material, and excessive consumption of resin material. In addition, the tip of the nozzle may cool, causing the fluidity of the molten-plasticized resin material to diminish, thereby adversely affecting injection, and causing the molded product to become uneven, or the like.
In order to prevent drooling , the plasticizing-measuring process is typically performed so that the nozzle at the tip of the heating cylinder abuts against the sprue of the mold (hereinafter, referred to as "nozzle touch") or the nozzle is closed by being retreated and separated from the mold.
When carrying out the plasticizing-measuring process in the nozzle touch state, the nozzle is closed by maintaining nozzle touch without separating the nozzle from the solidified resin material in the sprue that is injected in the preceding molding cycle.
On the other hand, conventional examples of closing the nozzle by retreating and separating the nozzle from the mold, are disclosed in Japanese patent publication No. Sho 49-2976, Japanese patent laid-open publication Nos. Sho 47-5690, Sho 59-7026, Utility Model laid-open publication Nos. Sho 49-146259, Hei 49-107015 and the like.
For instance, JP patent publication No. Sho 49-2976 discloses an automatic covering apparatus in which a ring holding frame member is fixed around an outer circumference of a heating cylinder. On a side of the ring holding frame member, an operation lever is pivotably provided through a first pivot pin. In a recessed portion of the operation lever, a nozzle cover is slidably supported through a second pivot pin and a driving mechanism for rotating the nozzle cover and the operation lever is provided.
In this embodiment, at an almost central portion of the nozzle cover, a recessed portion is configured so that its shape corresponds approximately to the shape of the tip of the nozzle. In addition, on a flange portion formed on the ring holding frame member, a regulating bolt is provided. The regulating bolt is adjusted so that the shape of the recessed portion of the nozzle corresponds to the shape of the tip of the nozzle.
In order to prevent molten resin from semi- or partially-solidifying in the nozzle due to cooling, a heat retaining ring is provided to heat the tip of the nozzle when the nozzle is separated from the mold. Additionally, when the nozzle is in contact with the mold, the heat retaining ring is separated from the tip of the nozzle. In this embodiment, after injection molding, while the mold is opened and the molded product is removed, the heat retaining ring is automatically mounted on the tip of the nozzle to surround and rapidly heat the tip of the nozzle. Similarly, during injection, immediately before the tip of the nozzle contacts the mold, the heat retaining ring separates and retreats from the tip of the nozzle. Thus, the molten resin material remains in a molten state and never semi-or partially- solidifies in the nozzle. As a result, the resin remains available for the next injection.
Of the above conventional techniques, when molding cycles are repeated by carrying out the plasticizing-measuring process in the nozzle touch state, and the temperature of the mold is set lower than that of the molten resin material, the nozzle can cool causing the resin material to solidify, thereby clogging the nozzle.
Further, when the temperature of the mold is set higher than that of the molten resin material, the temperature of the nozzle can increase due to heat transmitted from the mold. As a result, if the resin material being used is a thermosetting resin, the resin may be heated to such an extent that the resin begins to burn. In addition, if the resin material used is a thermoplastic resin, premature curing can occur before injection, causing the resin material to solidify and clog the nozzle.
In either case, continuous molding is extremely difficult.
In addition, there has been a problem that, if the plasticizing-measuring process is carried out in the nozzle touch state, that is, in the state where the nozzle is closed by resin material injected in the preceding molding cycle that has solidified in the sprue of the mold, until the plasticizing-measuring process is finished, the product molded in the preceding molding cycle cannot be removed. As a result, the duration of the molding cycle cannot be shortened.
In addition, of the aforementioned conventional techniques, in an apparatus wherein the nozzle is closed by being retreated and separated from the mold, as disclosed in JP patent publication No. Sho 49-2976, and the like, a problem occurs in that heat from the nozzle transmits to the nozzle cover decreasing the temperature of the nozzle and causing resin material therein to solidify or harden, thereby clogging the nozzle and making continuous molding impossible.
Further, of the above conventional techniques, in an apparatus wherein nozzle cooling is prevented, a tip of a nozzle is surrounded by a heat retaining ring to heat the nozzle, as disclosed in JP Utility Model laid-open publication HEI 3-64724. In such an apparatus, when the mold and nozzle are separated, the heat retaining ring surrounds and heats the tip of the nozzle. Also, in order to separate and retreat the heat retaining ring from the tip of the nozzle immediately before the tip of the nozzle contacts the mold, a divisional type heat retaining ring is provided, wherein no specific nozzle closing means is provided. As a result, drooling cannot be effectively prevented. Further, in this embodiment, "since the divisional heat retaining ring is needed to be rapidly heated, it is preferable to have 5 times or more heat capacitance compared with the fixed heater installed in the conventional nozzle" (See Hei at page 5, lines 1-4). In addition, this embodiment does not address problems such as deterioration in resin material quality where thermoplastic resins are used, and premature curing in the case where a thermosetting resin is used.
The present invention is directed to solving the above problems, by providing an injection molding machine that can effectively prevent leakage of resin material from a nozzle during a plasticizing-measuring process and can perform continuous molding while maintaining an amount of resin material in a molten state suitable for injection molding, so that molding efficiency can be enhanced.